System for, and method of, processing quadrature amplitude modulated signals

Abstract

Analog signals encoded with quadrature amplitude modulation (QAM) pass through a coaxial cable at a particular baud rate. These signals have a carrier frequency individual to the TV station being received. They are mixed with signals from a variable frequency oscillator to produce signals at a particular intermediate frequency (IF). An analog-digital converter (ADC) converts the IF signals to corresponding digital signals which are demodulated to produce two digital signals having a quadrature phase relationship. After being filtered and derotated, the digital signals pass to a symmetrical equalizer including a feed forward equalizer (FFE) and a decision feedback equalizer (DFE) connected to the FFE in a feedback relationship. The DFE may include a slicer providing amplitude approximations of increasing sensitivity at progressive times. Additional slicers in the equalizer combine the FFE and DFE outputs to provide the output data without any of the coaxial cable noise or distortions. The equalizer outputs and initially the derotation outputs, and the slicer outputs, servo (1) the oscillator frequency to obtain the IF frequency, (2) the ADC sampling clock to obtain the digital conversion at a rate related to the particular baud rate and (3) the derotator. The servos may have (1) first constants initially after a change in the station selection and (2) second time constants thereafter. The ADC gain is also servoed (1) initially in every ADC conversion and (2) subsequently in every nth ADC conversion where n=integer>1. The above recover the QAM data without any of the coaxial cable noise or distortions.

Description

[0001]This invention relates to systems for, and methods of recovering digitally modulated television signals from the noise and distortion in coaxial cables. More particularly, this invention relates to systems for, and methods of, recovering quadrature amplitude modulated signals from the noise and distortion in coaxial cables. In these systems and methods, quadrature amplitude modulation is used to transmit the television information. The systems and methods of this invention use digital techniques to recover the quadrature amplitude modulated signals from the noise and distortion in the coaxial cables.BACKGROUND OF THE INVENTION[0002]Modern digital telecommunication systems are operating at ever-increasing data rates to accommodate society's growing demands for information exchange. However, increasing the data rates, while at the same time accommodating the fixed bandwidths allocated by the Federal Communications Commission (FCC), requires increasingly sophisticated signal proc...

AI Technical Summary

Benefits of technology

[0006]This invention recovers the quadrature amplitude modulated data by using digital techniques. The current embodiment of the invention uses only three (3) integrated circuit chips to provide such recovery. The invention recovers the quadrature amplitude modulated data while eliminating substantially all of the noise and distortion in the coaxial cables. The invention also provides for an increased speed of operation, thereby being capable of handling television signals transmitted at increased baud rates. The three (3) integrated circuit chips consume a relatively low amount of power and occupy a relatively small space. Steps are now being taken to provide in a single chip the system now provided in three (3) chips. This chip will occupy even less space and consume less power than the three (3) chip system.

[0008]After being filtered and derotated, the two digital signals pass to a symmetrical equalizer including a feed forward equalizer (FFE) and a decision feedback equalizer (DFE) connected to the FFE in a feedback relationship. The DFE may include a slicer providing amplitude approximations of increasing sensitivity at progressive times. Additional slicers in the equalizer combine the FFE and DFE outputs to provide the output data without any of the coaxial cable noise or distortions.

Problems solved by technology

The three (3) integrated circuit chips consume a relatively low amount of power and occupy a relatively small space.

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